The federal government spent $7 billion to repair damage from tornadoes since 2001, and experts say the conditions for killer twisters, like the storms that ravaged Alabama over the weekend, might be sharper with rising temperatures.

The United States has a long history of deadly tornadoes, underscored by the EF4 twister that killed at least 23 people on Sunday in Lee County, Ala., about 110 miles south of Atlanta.

The National Weather Service reported that the tornado’s path of destruction stretched at least 24 miles, packing 170 mph winds and opening a milewide gash through forests, homes and buildings in Beauregard, Ala.

Few other tornadoes have claimed so many lives or caused so much damage so quickly. But it’s unclear if the violent storm cell that spawned the tornado carried the signature of climate change.

Experts say that remains an open and hotly debated question. No known scientific studies have established a cause-and-effect relationship between tornadoes and climate warming.

But scientists do say they are witnessing macro-scale changes in tornado frequency and variability across the United States, and an analysis by E&E News of federal disaster spending since 1954 shows that tornadoes are also becoming more costly.

The Federal Emergency Management Agency has spent $7 billion since 2001 on tornado-related damage, according to FEMA data. That includes $5.3 billion to repair roads, utilities and other public facilities and $1.7 billion paid to owners of damaged homes or businesses.

“The science is nascent, but early results suggest that tornadoes and severe convective storms that produce tornadoes will become more frequent, but also more variable,” said Walker Ashley, an atmospheric scientist at Northern Illinois University who specializes in risk and vulnerability to extreme weather events.

Ashley and Stephen Strader of Villanova University say in a 2016 article for the Bulletin of the American Meteorological Society: “Recent climate research has revealed an intensification in the year-to-year variability and clustering of tornado counts, as well as the potential for increasingly frequent and more variable environments supportive of severe convective storms and their hazards due to anthropogenic climate change.”

Much of the data analysis on U.S. tornadoes comes from NOAA’s National Severe Storms Laboratory in Norman, Okla. Harold Brooks, a senior scientist at the center, said decades of NWS data on tornadoes indicates that “things are different than they used to be,” but the causes of those changes remain unknown.

Among other findings, NOAA says that while the number of tornadoes rated EF1 or higher on the Enhanced Fujita scale has remained constant since the 1970s, at just under 500 events annually, the timing and frequency of tornadic activity are changing.

For example, in the 1970s, there were about 150 days per year with at least one confirmed tornado in the United States. That number fell to between 90 and 100 days in recent years, Brooks said. Over the same period, however, the number of days with dozens of tornadoes—30 or more—has increased by a factor of five, from one day every other year to 2.5 days annually.

Other studies show that the nation’s traditional “tornado alley” has shifted, with a slight increase in the number of tornadoes occurring in the Mid-South region and a decrease in western Oklahoma and the Texas Panhandle. “The changes are relatively small but statistically significant,” Brooks said.

Disaster data from FEMA show that tornado damage is concentrated in four Midwestern and Southeastern states. Iowa, Tennessee, Missouri and Alabama account for nearly half of the money FEMA has spent on tornado-related damage since 2001, E&E News found. A catastrophic EF-5 tornado that damaged 85 counties in Iowa in May 2008 cost FEMA $1.3 billion.

As the number of disaster-causing tornadoes has increased, so has their expanse. For example, in the 1980s, tornadoes damaged an average of nine counties per year, E&E News found. But in the decade from 2007 through 2016, tornadoes damaged an average of 21 counties annually.

Some of the hardest-hit counties are large population centers. Jefferson County, Ala., encompasses much of the Birmingham area and is the most populous county in Alabama. The county has experienced more tornadoes—16—than any other county since 1953, FEMA records show.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.